Results and Discussion Highly uniform PLGA particles with diameters in the range of 140-500 nm, 1000-fold smaller than those gen-erated by Quevedo et al., can be generated with the Fluid
Trang 1R E S E A R C H Open Access
Fabrication of PLGA nanoparticles with a fluidic nanoprecipitation system
Hui Xie, Jeffrey W Smith*
Abstract
Particle size is a key feature in determining performance of nanoparticles as drug carriers because it influences cir-culating half-life, cellular uptake and biodistribution Because the size of particles has such a major impact on their performance, the uniformity of the particle population is also a significant factor Particles comprised of the poly-mer poly(lactic-co-glycolic acid) (PLGA) are widely studied as therapeutic delivery vehicles because they are biode-gradable and biocompatible In fact, microparticles comprised of PLGA are already approved for drug delivery Unfortunately, PLGA nanoparticles prepared by conventional methods usually lack uniformity We developed a novel Fluidic NanoPrecipitation System (FNPS) to fabricate highly uniform PLGA particles Several parameters can
be fine-tuned to generate particles of various sizes
Background
Particles comprised of the polymer
poly(lactic-co-glyco-lic acid) (PLGA) are widely studied as therapeutic
deliv-ery vehicles because they are biodegradable [1] and
biocompatible [2-4] In fact, microparticles comprised of
PLGA are already approved for establishing sustained
release of leuprolide (Lupron Depot) and triptorelin
(Trelstar) Similar PLGA particles also show promise as
a delivery vehicle for proteins [5,6], siRNA [7], and for
presenting antigens to dendritic cells for vaccination
[8-10] It is also becoming clear that PLGA particles
offer considerable flexibility in choosing a route of
deliv-ery because they have proven to be effective when
injected intramuscularly [11,12], when delivered via
inhalation [13-15], and recent results indicate that they
also have promise for oral delivery of drugs and antigens
[16-19]
Particle size is one of the key features in determining
performance because it influences circulating half-life,
cellular uptake and biodistribution [20-22] The kinetic
aspects of drug release are also strongly influenced by
particle size [23-25] Early interest in drug-loaded
parti-cles centered on their application as vehiparti-cles for
sus-tained drug release, but now there is great interest in
using similar particles for targeting the delivery of drugs
to specific tissues, vascular beds, and cells For the latter
application smaller particles, particularly those in the range of ~100 nm, are likely to be advantageous because they are taken up by cells at rates 15 to 250 fold greater than micron size particles [26] This difference in the rate of uptake can be the distinction between specific and non-specific uptake For example, PLGA nanoparti-cles targeted to dendritic cells with an antibody are taken up specifically, but microparticles targeted with the same antibody are taken up non-specifically [8] The uniformity of the particle population is also a significant factor in performance Preparations of particles that are highly uniform will exhibit more consistent biodistribu-tion, cellular uptake, and drug release Preparations of particles lacking uniformity will exhibit variance in all of these parameters, making it difficult to draw conclusions about which subset of the particle population is respon-sible for biological effect
There are many different methods of fabricating solid polymeric particles Gas flow focusing [27] and electro-spray [28,29] can be used to fabricate PLGA microparti-cles with uniform sizes but these approaches have not been widely used to generate nanoparticles Several sol-vent-based methods can be used to make polymeric nanoparticles including interfacial polymerization [30], the evaporation of emulsions [31] and nanoprecipitation [32] In most cases however, these flow based approaches lack precise control at the macro level, so they yield particles with a broad size distribution Con-sequently, extra steps such as filtration or centrifugation
* Correspondence: jsmith@burnham.org
Sanford-Burnham Medical Research Institute, 10901 North Torrey Pines Road,
La Jolla, CA 92037 USA
© 2010 Xie and Smith; licensee BioMed Central Ltd This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and
Trang 2are required to isolate the population with the desired
size [33] One solution to this problem is the application
of microfluidic platforms, which provide extremely
pre-cise control over most aspects of the mixing and
preci-pitation process For example, Karnik et al developed
an elegant microfluidic system that precipitates PLGA
nanoparticles by focusing the flow of PLGA in organic
solvent by two intersecting streams of aqueous solvent
[34] With this approach highly uniform PLGA particles
with diameters of less than 50 nm could be fabricated
The use of microfluidic devices is not without
limita-tions though As Quevedo et al pointed out, such
devices require specialized fabrication procedures and
materials that are not widely available, and they can be
easily clogged by particle debris [30] As an alternative,
Quevedoet al proposed a rather simple fluidic system
capable of establishing flow conditions suitable for
pro-duction of monodisperse particles [30] The utility of
the device was demonstrated by using the device to
enact interfacial polymerization during flow to produce
hollow polyamide shells with diameters ranging from
300-800μm, depending on polymer concentration and
flow rates Here we show that a similar system, without
dramatic reductions in dimension, can be applied to
enact an entirely different process, nanoprecipitation
Results and Discussion
Highly uniform PLGA particles with diameters in the
range of 140-500 nm, 1000-fold smaller than those
gen-erated by Quevedo et al., can be generated with the
Fluidic Nanoprecipitation System (FNPS) The FNPS can be constructed with general lab equipment and sup-plies An inlet channel (26s needle) inserts into the cen-ter of a dispersing channel (Tygon tubing with ID 3/ 32’’) (Figure 1) Flow through each channel can be main-tained with peristaltic pumps A major advantage of this flow-based system is that all of the PLGA droplets are created from the end of the inlet channel under pre-cisely the same conditions (e.g flow rate, injection rate, polymer concentration,etc.)
Because the preparation and characterization of well-defined sizes of particles remain a challenge, the perfor-mance of this system was gauged by comparing PLGA particles fabricated using the FNPS (Figure 2A) to the conventional nanoprecipitation method (Figure 2B) Par-ticles fabricated by the FNPS have a diameter of 148 ±
14 nm, but particles fabricated by the conventional nanoprecipitation method, using the same solvents and polymer concentrations, are 211 ± 70 nm in diameter Importantly, the size uniformity of the PLGA particles fabricated using the FNPS is such that all the particles fall within the 100 to 190 nm diameter range, and 70% are between 130 and 160 nm; the particles fabricated using the conventional method have a much broader size distribution, with only 26% having a diameter of
190 to 220 nm (Figure 2C) In order to obtain nanopar-ticles with small size distribution from conventional nanoprecipitation methods, a filtration step is usually necessary; Gaumetet al reported that as much as 95%
of the particles can be lost during filtration [35] Because
Figure 1 A schematic of the Fluidic NanoPrecipitation System (FNPS) (A) Cartoon of FNPS Sample inlets are inserted into the dispersing channel The inlet channel contains PLGA polymer that precipitates upon contact with the surfactant in the dispersing channel, freezing the particles in a spherical morphology (B) Side view of the channel PLGA droplets are exposed to the hydrodynamic force of the continuous flow.
Xie and Smith Journal of Nanobiotechnology 2010, 8:18
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Trang 3of the small size distribution of the nanoparticles
gener-ated using FNPS, filtration is not required prior to use
The size of PLGA particles generated with the FNPS
can be changed by adjusting the flow rate of the
disper-sing phase For example, a shift from a flow rate of 35
ml/minute to 50 ml/minute and then to 80 ml/minute
decreased particle size from 327 ± 19 nm to 278 ± 35
and then to 193 ± 19 nm (Figure 3A) Similarly, a
decrease in PLGA concentration from 40 mg/ml to 20
mg/ml and then to 10 mg/ml resulted in a reduction in particle diameter from 393 ± 38 nm to 327 ± 19 nm to
231 ± 35 nm (Figure 3B) Since the FNPS is a water/ water miscible solvent system, the composition of the dispersing phase can also be used to control the size of the particles Increasing the concentration of methanol
in the dispersing phase from 20% to 50% and then to 80%, coincided with the reduction in particle size from
512 ± 45 nm to 315 ± 36 nm and then to 148 ± 14 nm
Figure 2 Highly uniform PLGA nanoparticles are fabricated by the Fluidic NanoPrecipitation System (FNPS) Scanning Electron Microscopy (SEM) images of PLGA nanoparticles fabricated by the (A) FNPS, or the (B) conventional nanoprecipitation method (C) Diameters of the particles were measured by using ImageJ For each sample, the mean diameter was calculated based on the measurements of 200 randomly chosen particles White bars indicate the distribution of diameters observed for PLGA nanoparticles fabricated by FNPS (average diameter 148 ±
14 nm) Black bars indicate the distribution of diameters for PLGA nanoparticles fabricated by the traditional nanoprecipitation method (average diameter 211 ± 70 nm) Samples were imaged without prior filtration.
Trang 4(Figure 4) These data suggest that by optimizing all
three of these parameters, the FNPS has the flexibility to
generate uniform particles across a wide range of sizes
from below 100 nm to above 1μm
The yield of particles is another important aspect of
any fabrication method We found that the yield of
par-ticles from the FNPS is typically 80% of the mass of the
PLGA in the inlet solution Consequently, under the
various conditions used for this study, the FNPS
gener-ated between two and eight mg of particles/ml/hr This
compares favorably with the yield of three mgs/ml/hr
fabricated using similar concentrations of PLGA by the
microfluidic system reported by Karniket al [34] The FNPS has many advantages including the ability to scale
up production by simply increasing the number of inlets entering the dispersing phase The dispersing stream could also be recirculated to increase the final concen-tration of particles in the fluid In addition, because the devise has a low risk of clogging, it can be used continuously
The mechanism by which the FNPS is able to generate such small and uniform particles is worthy of discussion One factor that influences the final size of the solidified particles is the size of the monodisperse droplets from
Figure 3 The diameter of PLGA nanoparticles can be controlled by the flow rates and PLGA concentrations (A) SEM images and diameters of PLGA nanoparticles fabricated at dispersing flow rates of 35 ml/min, 50 ml/min, and 80 ml/min (B) SEM images and diameters of PLGA fabricated at PLGA concentrations of 10 mg/ml, 20 mg/ml, and 40 mg/ml Diameters were measured by using ImageJ For each sample, the mean diameter was calculated based on the measurements of 100 randomly chosen particles Samples are imaged without filtration.
Xie and Smith Journal of Nanobiotechnology 2010, 8:18
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Trang 5which they are precipitated Quevedoet al [30]
demon-strated that the flow in a fluidic system with dimensions
similar to that used here is comparable to a traditional
microfluidic system They also found that a higher
Rey-nolds number favors the formation of smaller droplets
So then, parameters like the flow rate in the dispersing
channel, and the liquid composition within that channel
will impact Reynolds number and can be used to
con-trol the size of droplets These conclusions are entirely
consistent with our observation that the flow rate alters
the final particle size
The actual process of nanoprecipitation will also
influ-ence particle size This is how our approach differs from
that of Quevedo et al [30] They used the T-junction
system to assist in the precipitation of emulsions that
were subsequently made solid by interfacial
polymerization via the action of a cross-linker in the dis-persing channel This process creates “hollow” particles with diameters of several hundred microns In contrast,
we directly precipitated the PLGA polymer by rapid sol-vent exchange, also called nanoprecipitation [32] The mechanism of particle formation during nanoprecipita-tion is not entirely understood, meaning that the precise outcome cannot be predicted Nevertheless, as has been previously discussed [32], nanoprecipitation appears to
be governed by the Marangoni effect, wherein move-ment in an interface is caused by longitudinal variations
of interfacial tension [36] In such a case, precipitation
is driven by i) solute transfer out of the phase of higher viscosity, which is influenced by high concentration gra-dients at the interface; and ii) by interfacial tension, which, in the case of the FNPS, is determined by
Figure 4 The diameter of PLGA nanoparticles can be controlled by varying the methanol concentrations (v/v) in the dispersing phase Diameter of PLGA nanoparticles fabricated using 20%, 50% or 80% v/v methanol in the dispersing phase of the FNPS The flow rate of the dispersing channel was maintained at 50 ml/minute Samples were imaged by SEM without prior filtration The diameter of the particles was calculated by using ImageJ For each sample, the mean diameter was calculated based on the measurements of 100 randomly chosen particles.
Trang 6turbulence resulting from flow in the dispersing channel.
Consequently, the size of the final particle will be
influ-enced not only by features of the dispersing channel
related to Reynolds number, but also by factors that
influence interfacial tension These include the polymer
concentration, the presence and concentration of
surfac-tant [37], and the nature of any payload that is
co-preci-pitated into the particles [37] The depth of insertion of
the inlet into the dispersing channel might also
influ-ence particle size and geometry due to altered
turbu-lence However, with this prototype FNPS, it was
impossible to test this possibility because we could not
control the depth of insertion with great precision
Conclusions
In summary, the FNPS described here provides an
approach to produce very small and highly uniform
polymeric particles, in the absence of sophisticated
instrumentation or a microfluidic system The particles
are suitable for multiple uses including drug and
ima-ging agent encapsulation
Materials and methods
Materials
PLGA Resomer RG502H was purchased from
Boehrin-ger-Ingelheim (Ingelheim, Germany) PLGA sample
solutions were prepared by dissolving PLGA in
acetoni-trile For example, a 40 mg/ml PLGA solution was
pre-pared by dissolving 40 mg RG502H in 1 ml acetonitrile
Polyvinyl alcohol (PVA, 87%-89% hydrolyzed) was
pur-chased from Sigma-Aldrich 1% PVA solution was
pre-pared by dissolving 1 g PVA in 100 ml DI water at
room temperature and filtered to remove any particulate
matter
Device fabrication and experimental setup
A Fluidic NanoPrecipitation System (FNPS) was
fabri-cated by inserting a stainless steel needle (Hamilton
HA-91039 26s syringe needle) with an inner diameter
0.11 mm, into a Tygon® tubing (ID 3/32’, OD 5/32’)
that was used to pass the dispersing phase The needle
was inserted to the interior at 50% of the tubing
diameter
The PLGA solution fed into the dispersing channel
with a 3 ml syringe controlled by a single syringe pump
(KDS100, KD Scientific, Massachusetts, USA) A stream
of surfactant (1% PVA solution, 20 ml) passing through
the dispersing channel (Tygon® tubing with ID 3/32’,
and OD 5/32’) was controlled by a Fisher Scientific
Variable-Flow Peristaltic Pump
Nanoparticles were prepared starting with 10 and 40
mg/ml of PLGA RG502H polymers in acetonitrile
Sam-ples (0.2 ml) were injected at a flow rate of 3.2μl/min
Nanoparticles were collected into a beaker for analysis
The nanoparticles were washed by centrifuging for 15 minutes using an Eppendorf 5415R at 13200 rpm at room temperature and then removing the supernatant The nanoparticles were resuspended in DI water by bath sonication (Branson’s Model B200) This was repeated three times and the final suspension was sent for analysis Scanning Electron Microscope (SEM)
SEM experiments were conducted by depositing the nanoparticle suspension on freshly cleaved mica and allowing them to dry A thin film of Au was sputtered onto these mica substrates with sample Samples were imaged with scanning electron microscopy (SEM; JEOL 5800LV) without filtration or purification Particle size was measured by using ImageJ For each sample, the mean diameter was calculated based on the measure-ments of 100 randomly chosen particles
Acknowledgements The work described in this manuscript was supported by a grant from the U.S National Institutes of Health (HL080718) awarded to JWS.
Authors ’ contributions JWS and HX conceived and designed the experimental strategy and interpreted the findings.
HX performed all experiments All authors read and approved the final manuscript.
Competing interests The authors declare that they have no competing interests.
Received: 15 March 2010 Accepted: 13 August 2010 Published: 13 August 2010
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doi:10.1186/1477-3155-8-18 Cite this article as: Xie and Smith: Fabrication of PLGA nanoparticles with a fluidic nanoprecipitation system Journal of Nanobiotechnology
2010 8:18.
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